Japanese Laid-Open Patent Application No. 2008-135781 suggests a semiconductor package, which is formed by arranging a semiconductor element between upper and lower substrates and filling a mold resin in the circumference of the semiconductor element. The upper substrate is fixed to the lower substrate via spacer members to maintain an interval between the upper and lower substrates, and the mold resin is filled between the upper and lower substrates. The upper substrate and the lower substrate are electrically connectable through the spacer members. External connection terminals are provided on the backside of the lower substrate so that electronic parts can be mounted on the upper substrate.
In the semiconductor package having the above-mentioned structure, it is suggested to use a copper-core solder ball as a spacer member. The copper-core solder ball includes a copper-core ball and a solder covering the circumference of the copper-core ball. The solder serves as a joining material, and the copper-core ball serves as a spacer. That is, connection pads of the upper substrate and connection pads of the lower substrate are joined by solder by reflowing the solder in a state where the copper-core solder balls are sandwiched between the connection pads of the upper substrate and the connection pads of the lower substrate, respectively. The copper-core balls inside solder are set in a state where the copper-core balls are sandwiched between the upper substrate and the lower substrate. Thereby, each of the copper-core balls can serve as a spacer. Therefore, a distance between the upper substrate and the lower substrate is determined according to the size of the copper-core balls, and can be maintained constant.
After connecting the upper substrate and the lower substrate using the copper-core solder balls, the mold resin is filled between the upper substrate and the lower substrate so that the upper substrate and the lower substrate are fixed surely to each other. Thus, the semiconductor element and the copper-core solder balls mounted on the lower substrate are embedded in the mold resin.
When the upper substrate and the lower substrate are connected and fixed by copper-core solder balls as mentioned above, the copper-core balls and the solder are confined within the mold resin. When reflowing solder balls, which serve as external connection terminals of the semiconductor package, the solder in the circumference of the copper-core balls inside the mold resin is also melted due to heat applied by the reflow.
If the copper-core balls are heated at a temperature at which the solder is melted in the circumference of the copper-core balls, the copper-core balls and the solder are subjected to thermal expansion, and, thereby, a volume thereof tends to increase. However, because the copper-core balls and the solder are confined within the mold resin, the pressure of the melted solder is increased. Then, if there is a portion where adhesion is weak in portions where the mold resin adheres to the upper substrate and the lower substrate, the melted solder may intrude into the weak-adhesion portion.
For example, if there is such a weak-adhesion portion between the mold resin and a solder resist on the substrate or between the solder resist and a wiring pattern, the melted solder intrudes into the weak-adhesion portion while pealing the mold resin or the solder resist. If the solder intrudes along the solder resist or the wiring pattern, adjacent connection pads may be short-circuited or the wiring pattern may be short-circuited due to the intruding solder.
Thus, it is desirable to develop a technique according to which a melted solder within a mold resin does not cause the above-mentioned problem even if an upper substrate and a lower substrate are connected using a copper-core solder ball as a spacer member.